Vertically arrayed loudspeakers systems, or “line arrays”, are currently the predominant form of system used in large and medium scale touring sound systems. A typical line array is shown in FIG. 1. The line array 1 comprises several loudspeaker elements 2 arranged vertically by being suspended from the ceiling of a venue on suspension chains 3. As shown in FIG. 2, the splay angle xi between neighbouring elements is adjusted by means of an adjustment bar 4 which allows different settings for the spacing between the back of the elements, while the distance between the front of the elements remains fixed.
Due to the complex nature of the interactions of the elemental loudspeakers a wealth of CAD tools is available that predict the output of a given array or combination. Such tools include EASE, form ADA (www.ada-library.de), CATT, from CATT-Acoustic (http://www.catt.se/), and DISPLAY, from Martin Audio (www.martin-audio.com.). These kind of systems have been available for at least 10 years.
To design an array with these tools the user manually alters the splay angles between one element speaker and the next in the array and inspects the output; this process is repeated until an acceptable output is achieved.
Some of the factors important to the success of the manual method of array design are:    1. Accuracy of the radiation model used to predict the output of the array.    2. The user's mental model of how the complete sound system works.    3. Speed of feedback to the user of the CAD system's prediction.    4. Size and granularity of the domain used.    5. Time available for the user to find a solution using the CAD system.
The present invention aims to improve upon this method of configuring speaker arrays for use so that they provide the desired sound field.
The simple radiation model that forms the basis for practically all array CAD tools has been termed the directional point source model. Pressure at the receiver points r is formed from the complex summation of pressure from all elemental sources. Each elemental source has an associated measured complex ‘balloon’ of pressure at a set of frequencies f and an orientation. The computation defines a ray from each source to each receiver point, for each frequency the pressure where the ray intersects the balloon is determined via a complex interpolation of nearby measured points, and this pressure is then propagated to the receiver points to provide a pressure amplitude P(r,f) (H. Staffeldt. Prediction of sound pressure fields of loudspeaker arrays from loudspeaker polar data with limited angular and frequency resolution—108th Convention of the Audio Engineering Society, Preprint : 5130, 2000). It is assumed that the measured data for each source, which is used to determine the ‘balloon’, is obtained in the far-field for that source and that the presence of neighbouring enclosures is not significant, since neighbouring enclosures are seldom present when source measurements are performed. Despite the latter assumption the simple model is thought to give a good indication of the expected pressure at the receiver points.
Many users of CAD systems tasked with manual design of vertical arrays only evaluate the performance on a thin strip of the venue normal to the front of the array. This method allows relatively rapid feedback when compared to full audience plane calculations and it is found that good performance on the strip generally reflects in good performance in the full calculation assuming each element has consistent horizontal directionality. The examples of the invention given below follow this convention but the invention also allows for the incorporation of points outside the strip whilst still avoiding a full calculation.
The normal presentation of performance along the strip to the user either employs overlaid frequency responses at different receiver points or overlaid plots of pressure at the receiver points for different frequencies (distance plots). Both of these views become cumbersome when more than 10 graphs are overlaid and unworkable when frequency and receiver points are of the order of 100 or more. What is required is to be able to view the pressure at all receiver points and at all frequencies at once. This can be achieved with a 3D plot where the x axis represents frequency, the y axis receiver position index and the z axis pressure. Conceptually it is like stacking up all the frequency response plots along the y axis or indeed all the distance plots along the x axis. In this manner the result of any change is seen across the entire range of frequency and position. Such 3D plots are shown in the Figures, discussed below, for the performance of the speaker arrays of the invention.
EP 1 523 221 A2 discloses a system for setting up a domestic hi-fi system, in particular the sub woofers thereof. A sub woofer is placed in possible positions and the transfer function of the system is measured by sampling a test sound with a microphone at one or plural listening positions. The number of available transfer functions is increased by modifying the measured ones with ones for adding delay to the sound signal before it is reproduced by the sub woofer etc. Set-ups having more than one subwoofer are made by superposing the transfer functions. The system does not therefore model the propagation of the sound in the space but merely measures the output (i.e. the sound at the listening position) empirically. The system searches through the possible systems and ranks them by various aspects of their transfer function, allowing one to be chosen.
GB 2 259 426 A discloses another audio system whose performance is empirically measured. An array of speakers is used to produce constant directivity over a wide range of frequencies. The directivity functions between each speaker of the array and each of a number of positions equidistant from the array are measured and then compensating filter functions are calculated, which filter functions are used by digital filters respective to the speakers that modify the otherwise common sound signal before it is applied to the individual speakers.